linux-kernel - Re: [RFC][PATCH 0/5] Optimize percpu-rwsem

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Message-ID: <20150605233622.GA31034@redhat.com>
Date:	Sat, 6 Jun 2015 01:36:22 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Tejun Heo <tj@...nel.org>, Ingo Molnar <mingo@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	der.herr@...r.at
Subject: Re: [RFC][PATCH 0/5] Optimize percpu-rwsem

On 06/05, Al Viro wrote:
>
> On Fri, Jun 05, 2015 at 11:08:57PM +0200, Oleg Nesterov wrote:
> > On 06/05, Al Viro wrote:
> > >
> > > FWIW, I hadn't really looked into stop_machine uses, but fs/locks.c one
> > > is really not all that great - there we have a large trashcan of a list
> > > (every file_lock on the system) and the only use of that list is /proc/locks
> > > output generation.  Sure, additions take this CPU's spinlock.  And removals
> > > take pretty much a random one - losing the timeslice and regaining it on
> > > a different CPU is quite likely with the uses there.
> > >
> > > Why do we need a global lock there, anyway?  Why not hold only one for
> > > the chain currently being traversed?  Sure, we'll need to get and drop
> > > them in ->next() that way; so what?
> >
> > And note that fs/seq_file.c:seq_hlist_next_percpu() has no other users.
> >
> > And given that locks_delete_global_locks() takes the random lock anyway,
> > perhaps the hashed lists/locking makes no sense, I dunno.
>
> It's not about making life easier for /proc/locks; it's about not screwing
> those who add/remove file_lock...

I meant, seq_hlist_next_percpu() could be "static" in fs/locks.c.

> And no, that "random lock" isn't held
> when modifying the (per-cpu) lists - it protects the list hanging off each
> element of the global list, and /proc/locks scans those lists, so rather
> than taking/dropping it in each ->show(), it's taken once in ->start()...

Sure, I understand. I meant that (perhaps) something like

	struct {
		spinlock_t lock;
		struct list_head *head
	} file_lock_list[];


	locks_insert_global_locks(fl)
	{
		int idx = fl->idx = hash(fl);
		spin_lock(&file_lock_list[idx].lock);
		hlist_add_head(...);
		spin_unlock(...);
	}

seq_hlist_next_percpu() could scan file_lock_list[] and unlock/lock ->lock
when it changes the index.

But please forget, this is really minor. Just I think that file_lock_list
is not actually "per-cpu", exactly because every locks_delete_global_locks()
needs lg_local_lock_cpu(fl->fl_link_cpu) as you pointed out.

Oleg.

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